Third-harmonic-generation spectroscopy of poly(p-phenylenevinylene): A comparison with oligomers and scaling laws for conjugated polymers

Abstract

The third-order nonlinear optical susceptibility ͑3͒ ͑Ϫ3;,,͒ of thin films of poly͑p-phenylenevinylene͒ ͑PPV͒ and several corresponding oligomers ͑OPV-n͒ has been investigated by third-harmonic generation using variable laser wavelengths from 900 to 1520 nm. The oligomers show a single three-photon resonance of ͑3͒ ͑Ϫ3;,,͒ which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the ͑3͒ spectrum of PPV. They are assigned to three-photon resonances with the maximum of the exciton absorption and with the threshold of the continuum of states, which can be located at 3.2Ϯ0.1 eV. This corresponds to an exciton binding energy of 0.7Ϯ0.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated -electron systems in their neutral form. Their ͑3͒ values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship(3) /␣ max ϳ max x , where ␣ max and max denote the absorption coefficient and wavelength of the low-energy absorption maximum. We obtain an exponent xϭ10Ϯ1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental results are discussed.

Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV

“…The so-called three-photon resonance occurs if the laser wavelength λ L is close to 3λ max . 10,18 There, (3) approaches the maximum value max (3) . We have studied the resonance behavior of (3) (-3ω;ω,ω,ω) using tunable laser wavelengths in the near-infrared.…”

“…Short chains such as conjugated oligomers show a significant increase in the wavelength of the maximum of the main absorption band (λ max ) and the second hyperpolarizability (γ) per repeat unit, which usually follow power law dependencies of the chain length in a regime up to approximately 2 nm. 10,11,18,51 For chain lengths larger than typically 2 nm, socalled saturation effects are observed, which means that λ max and γ per repeat unit converge to finite values that are sensitively dependent on the chemical structure and conformation of the polymer. [52][53][54][55][56][57][58][59][60] However, it was noted recently that the twophoton absorption coefficient of thin films of a polydiacetylene can still increase with M w , even if M w is significantly larger than the saturation limit of γ per repeat unit.…”

Effect of Donor−Acceptor Substitution on the Nonlinear Optical Properties of Oligo(1,4-phenyleneethynylene)s Studied by Third Harmonic Generation Spectroscopy

“…These data reveal a superlinear increase of |γ res | with L, which has similarly been observed in the cases of other conjugated oligomers such as oligophenylenes, 25 oligothiophenes, 26,27 and oligo-p-phenylenevinylenes. 16 The dashed line shown in the figure represents an increase from 3a to 3b according to a power law γ res ∼ L 3.3 .…”

“…The dispersion of the absorption coefficients R(λ) and the refractive index n(λ) of ultrathin films (d ∼ 50 nm) were determined from the transmission and reflection spectra measured with a UVvis-NIR spectrometer (Lambda 900) as described in earlier work. 16 The λ max and absorption coefficient R max data of the thin films are given in Table 1. They show spectroscopic data similar to those obtained for the CHCl 3 solutions.…”

“…16 The results are compared with our previous study 5 of nonlinear optical properties of a series of oligo(2,5-dipropoxy-1,4-phenyleneethynylene)s 3 5 and conclusions on the influence of the donor-acceptor substitution are made.…”